Various pneumatic tires have been proposed which contain a built-in sealant layer based upon a depolymerized butyl rubber layer.
For example, in U.S. Pat. No. 4,895,610 it is proposed to build a layer of butyl rubber-based composition into a tire which contains a polyisobutylene polymer and an organoperoxide, following which under a condition of elevated temperature used to cure the tire, a major portion of the butyl rubber is presented as being depolymerized to form a tacky material which has puncture sealant properties.
U.S. Pat. Nos. 4,228,839, 4,171,237 and 4,140,167 also relate to a sealant layer in a tire formed by depolymerizing and crosslinking a butyl rubber-based rubber composition, particularly by irradiation treatment.
U.S. Patent Application Publication No. 2005/0034799 A1 relates to a tire with built-in sealant comprised of a partially depolymerized butyl rubber via an organoperoxide which contains a dispersion of particulate pre-cured rubber particles.
U.S. patent application Ser. No. 10/368,259, filed Feb. 17, 2003, relates to a tire with built-in sealant comprised of a partially depolymerized butyl rubber via an organoperoxide which contains a particulate filler comprised of carbon black and/or coal dust and, optionally short fibers, hollow glass microspheres and rubber processing oil wherein the sealant may also contain a liquid diene-based polymer.
U.S. Patent Application Publication No. 2005/0034799 A1 relates to a self sealing tire containing a sealing material which may be contained within the tire and which may be comprised of a thermally degraded butyl rubber with an organic peroxide and containing a reinforcing filler.
U.S. Patent Application Publication No. 2005/0113502 A1 relates to a tire with a built-in colored sealant layer.
Additional patents which relate to various tire constructions which may involve built-in or built-on sealants for tires such as for example, U.S. Pat. Nos. 1,239,291, 2,877,819, 3,048,509, 3,563,294, 4,206,796, 4,286,643, 4,359,078, 4,444,294, 4,895,610, 4,919,183 and 4,966,213.
Further, U.S. Pat. No. 6,767,937 and U.S. Patent Application Publication No. 2003/0087998 A1 relate to various 2,2,6,6-tetra alkyl piperidine hindered amines for use in stabilizing various cellulose ester based polymer coatings and films (said U.S. Pat. No. 6,767,937) and for light stabilizing various thermoplastic organic polymers selected from polyolefins and copolymers and blends thereof (said U.S. Patent Application Publication No. 2003/0087998 A1).
In one aspect, the various built-in sealant layers for the pneumatic tires which are derived from a depolymerization of a butyl rubber-based sealant precursor composition typically contain a rubber reinforcing carbon black filler and are therefore black in color.
For this invention, it is desired to provide a built-in sealant layer for a pneumatic tire which is derived from a depolymerization of a butyl rubber-based sealant precursor composition which is of a black color (resulting from an inclusion of carbon black) or a color other than black (resulting from an inclusion of a colorant with a minimal, if any carbon black content), where the sealant of other than black color may be used as an aid to identify a puncture wound in a carbon black reinforced innerliner, tread and/or sidewall of said tire, depending somewhat upon the position of the built-in sealant layer, because of its contrasting color. Therefore, in one aspect, it is envisioned that such a non-black colored sealant layer is substantially exclusive of carbon black and particularly substantially exclusive of a rubber reinforcing carbon black. By the term “substantially exclusive of” carbon black it is intended that only a minimal amount, preferably none, of carbon black is present such as, for example, an impurity amount (e.g. less than about 0.1 phr) of carbon black, alternately an amount of up to about 2, preferably less than about 0.5, phr of carbon black, may be present so long as the sealant layer is of a color other than black and preferably contrasting with black colored (carbon black reinforcement containing) tire components which adjoin the built in sealant layer such as, for example, a tire innerliner component.
In practice, built-in sealants for tires based upon organoperoxide depolymerized butyl rubber have heretofore, in general, relied upon containing rubber reinforcing carbon black to provide a degree of reinforcement and thereby a suitable rubber viscosity to enable the sealant precursor composition to be conveniently processable both by formation into a suitable rubber sheet whether by extrusion or calendering and, also, by handling to build a sheet, or layer, of the sealant precursor composition into a tire assembly. Further, the presence of the rubber reinforcing carbon black is relied upon to provide a suitable viscosity of the built-in sealant in order to promote the stability of the sealant composition.
However, for this invention, synthetic, amorphous silica (aggregates of primary silica particles), preferably in the form of a precipitated silica, may be used as a primary reinforcing filler for the butyl rubber-based sealant precursor composition instead of rubber reinforcing carbon black where it is desired that the built in sealant is of a color other than of a black color.
In another aspect of the invention, a portion of the precipitated silica may be replaced by fillers such as, for example, clay, for example kaolin clay and calcium carbonate for which, therefore, the butyl rubber-based sealant precursor may contain one or more of such additional fillers in addition to the precipitated silica.
Significant challenges are presented in the use of the synthetic, amorphous silica, particularly precipitated silica, as well as the inclusion of the aforesaid additional fillers.
For example, contrary to rubber reinforcing carbon black, such precipitated silica, (e.g. precipitated silica aggregates of primary silica particles), normally contains a significant presence of water of hydration, and/or water of association on its surface as well as a significant presence of hydroxyl groups (e.g. silanol groups). The clay and calcium carbonate may also contain an associated water moiety.
While the mechanism may not be fully understood, it is envisioned that at least a portion of such water, and possibly a portion of such hydroxyl groups, is available to prematurely decompose the organoperoxide intended to be used to depolymerize the butyl rubber in the sealant precursor composition.
It is considered herein that such premature decomposition of the organoperoxide, whether by presence of the water of hydration and/or association or by the presence of the hydroxyl groups, may adversely affect the free radical generating activity of the organoperoxide for a timely depolymerization of the butyl rubber after the sealant precursor layer is built into the tire assembly and the tire is cured at an elevated temperature. Such prematurely formed free radicals may become associated with the silica, and possibly the clay and calcium carbonate, rather than be involved with the desired depolymerization of the butyl rubber. It is further envisioned that if excess organoperoxide is added to allow for such premature decomposition, a portion of such excess organoperoxide may later or otherwise gradually become available to interact with associated rubber layers of the tire assembly, or construction, adjacent to the built-in sealant.
In order to inhibit, retard and/or significantly prevent significant contact of such water moieties and hydroxyl groups of the amorphous silica aggregates with the organoperoxide, the synthetic amorphous silica may be treated in situ within the rubber composition prior to addition of the organoperoxide, or may be pre-treated prior to its addition to the rubber composition with a low molecular weight polyalkylene oxide polymer, which might sometimes be referred to as a polyalkylene glycol; with an alkylsilane, a coupling agent having a moiety reactive with the hydroxyl groups on the silica and another moiety which is normally interactive with carbon-to-carbon double bonds of an elastomer or by a combination of alkylsilane (e.g. alkoxysilane) and coupling agent.
An important aspect of this invention is providing a pneumatic tire with a built-in black or non-black colored sealant layer positioned (sandwiched) between dimensionally stable sulfur vulcanized rubber layers (e.g. between a tire inner liner rubber layer and tire carcass) which promotes sealing against an externally puncturing object to prevent, or significantly retard, leakage of gas (e.g. air) from the tire cavity through the puncture to the outside of the tire by allowing the pressure within the tire cavity (air pressure greater than ambient atmospheric pressure) to press the sealant against the puncturing object or the cavity caused by the puncturing object.
In the description of this invention, the term “phr” is used to designate parts by weight of an ingredient per 100 parts of elastomer unless otherwise indicated. The terms “elastomer” and “rubber” are used interchangeably unless otherwise indicated. The terms “cure” and “vulcanize” are used interchangeably unless otherwise indicated.